1. Field of the Invention
The invention generally relates to memory technology. In particular, the invention relates to non-volatile magnetic memory.
2. Description of the Related Art
Computers and other digital systems use memory to store programs and data. A common form of memory is random access memory (RAM), such as dynamic random access memory (DRAM) devices and static random access memory (SRAM) devices. DRAM devices and SRAM devices are volatile memories. A volatile memory loses its data when power is removed. For example, when a conventional personal computer is powered off, the volatile memory is reloaded through a boot up process. In addition, certain volatile memories such as DRAM devices require periodic refresh cycles to retain their data even when power is continuously supplied.
In contrast to the potential loss of data encountered in volatile memory devices, nonvolatile memory devices retain data for long periods of time when power is removed. Examples of nonvolatile memory devices include read only memory (ROM), programmable read only memory (PROM), erasable PROM (EPROM), electrically erasable PROM (EEPROM), flash memory, and the like. Disadvantageously, conventional nonvolatile memories are relatively large, slow, and expensive. Further, conventional nonvolatile memories are relatively limited in write cycle capability and typically can only be programmed to store data about 10,000 times in a particular memory location. This prevents a conventional non-volatile memory device, such as a flash memory device, from being used as general purpose memory.
An alternative memory device is known as magnetoresistive random access memory (MRAM). An MRAM device uses magnetic orientations to retain data in its memory cells. Advantageously, MRAM devices are relatively fast, are nonvolatile, consume relatively little power, and do not suffer from a write cycle limitation. There are at least three different types of MRAM devices, including giant magneto-resistance (GMR) MRAM devices, magnetic tunnel junction (MTJ) or tunneling magneto-resistance (TMR) MRAM devices, and pseudo spin valve (PSV) MRAM devices. GMR MRAM devices separate at least two ferromagnetic layers with a metallic layer. In a MTJ MRAM device, at least two ferromagnetic layers are separated by a thin insulating tunnel barrier. A PSV MRAM device uses an asymmetric sandwich of the ferromagnetic layers and metallic layer as a memory cell, and the ferromagnetic layers do not switch at the same time.
In a conventional fabrication process, layers of materials that form a magnetoresistive sandwich for an MRAM cell body are formed by depositing a relatively large sheet of magnetoresistive materials. Conventional processes then selectively remove portions of the deposited sheet to form the MRAM cell bodies. Preferably, chemical etching techniques are used to selectively remove portions of deposited sheets. Examples of chemical etching techniques include dry etching techniques and wet etching techniques. However, such chemical etching techniques are not applicable to the fabrication of MRAM cells because the materials that are used to fabricate MRAM cells are relatively difficult to etch away with chemicals. For example, copper (Cu) is relatively difficult to remove by a chemical etching process.
Those in the art have resorted to ion beam milling or ion beam etching (IBE) processes to remove magnetoresistive materials from undesired areas. Ion beam milling is a physical milling process. A resist material is applied to regions that will form cells to protect or mask the regions from the effects of ion beam milling. Areas that are not protected by the resist are removed from the substrate assembly by bombardment with ions. The bombardment of ions sputters or peels away the unprotected material from the substrate assembly. Disadvantageously, ion beam milling operates with relatively low selectivity, and the portions of the substrate assembly that are near to the edges of the photoresist or the boundaries of an MRAM cell body can be easily damaged. The damage can result in a cell in which an edge of the cell and a center of the cell do not switch in unison. In addition, ion beam milling etch rates are relatively low, which results in relatively high costs and relatively low throughput rates.
A technique is needed to form MRAM cells without the disadvantages of ion beam milling.
The invention relates to processes that advantageously form MRAM cells without the application of an ion bean milling process. Conventional processes rely on relatively slow and potentially harmful ion beam milling processes to remove materials from a magnetoresistive sandwich from areas proximate to other areas that will later form MRAM cell bodies. By contrast, a process according to an embodiment of the invention forms a layer of photoresist over areas other than those areas that correspond to MRAM cell bodies. The photoresist is removed by a lift-off process after the deposition of a magnetoresistive sandwich that forms the MRAM cell bodies, thereby safely removing the magnetoresistive sandwich from undesired areas while maintaining the magnetoresistive sandwich in the areas that will eventually become MRAM cell bodies.
One embodiment according to the invention is a process that produces a cell body in a magnetoresistive random access memory (MRAM). Advantageously, the process can produce either GMR or TMR cell bodies without resorting to ion beam milling processes.
The process begins by forming an insulating layer, such as a layer of silicon nitride (Si3N4), on a substrate assembly. The process patterns a trench in the insulating layer, which is adapted to accommodate a cell body that is to be formed. The substrate assembly can include a word line below the trench. In one embodiment, the trench further includes a window or an opening that allows a cell body that is later formed to make electrical contact with the word line.
The process forms a layer of photoresist over the insulating layer. The process removes portions of the photoresist layer that cover an area where the cell body is to be formed. A magnetoresistive sandwich is then formed on the substrate assembly. The magnetoresistive sandwich can be formed by physical vapor deposition (PVD) processes, chemical vapor deposition processes, and the like. The undesired portions of the magnetoresistive sandwich are removed from the substrate assembly by removing the remaining portions of the photoresist layer.
Another embodiment according to the invention is a process that forms magnetoresistive cell bodies in an insulating material, such as an insulating layer on a substrate assembly. The process begins by forming trenches in an insulating material. The bottom surfaces of the trenches correspond to first regions of the insulating material, where magnetoresistive cell bodies are to be formed. The process applies photoresist to second regions of the insulating material. The second regions correspond to areas where layers that form the magnetoresistive sandwich are not intended to remain. The process proceeds to form a sandwich of magnetoresistive materials on both the first regions and the second regions. Advantageously, the process then removes the sandwich of magnetoresistive materials from the second regions without resorting to an ion beam milling process. The process removes the sandwich of magnetoresistive materials from the second regions by removing the photoresist that had been applied to the second regions. A variety of processes, such as photoresist stripping processes and micromachining processes can be used to remove the photoresist.
Another embodiment according to the invention includes a partially completed magnetoresistive random access memory (MRAM). The partially completed MRAM includes a substrate assembly, an insulating layer on a top surface of the substrate assembly, a trench formed in the insulating layer, and a photoresist layer formed on the insulating layer. In one embodiment, the substrate assembly includes a word line that has been formed beneath the trench. The trench is adapted to accommodate at least one magnetoresistive cell body.
Another embodiment according to the invention includes a partially completed magnetoresistive random access memory (MRAM). The partially completed MRAM includes a substrate assembly, an insulating layer on a top surface of the substrate assembly, at least one trench, and a photoresist layer on a top surface of the insulating layer. The photoresist layer does not cover the bottom surface of the trench, which is adapted to accommodate at least one magnetoresistive cell body.
Another embodiment according to the invention includes a partially completed magnetoresistive random access memory (MRAM). The partially completed MRAM includes a substrate assembly, an insulating layer formed on a top surface of the substrate assembly, at least one trench, a photoresist layer, and a magnetoresistive sandwich. The trench is formed in the insulating layer and is adapted to accommodate at least one magnetoresistive cell body. The photoresist layer is present on a top surface of the substrate assembly, but is not present on a bottom surface of the trench. The magnetoresistive sandwich is formed on the photoresist layer and on the bottom surface of the trench.